Ultrasensitive Strain Sensor Research Articles

Graphene-Based Ultrasensitive Strain Sensors

We performed density function theory calculations to study the effect of strain on electronic properties of pristine and doped graphene. A dramatic change in density of states (DOS) and band struct...

A novel combination of graphene and silver nanowires for entirely stretchable and ultrasensitive strain sensors: sandwich-based sensing films

A great deal of engineering effort is focused on developing stretchable strain sensors for human motion monitoring and wearable devices. The ultrasensitivity and fast response under tiny strain (1%) while maintaining the working range remain the grand challenge. In this work, we propose an entirely stretchable strain sensor based on the sandwich sensing film, which is fabricated by vacuum filtration of silver nanowires (AgNWs)/ graphene/ AgNWs in sequence and the injection of liquid metal as electrodes. The novel sandwich sensing film endows the stretchable strain sensor high sensitivity under tiny strain (Gauge factor = 111.5 at 1%), fast response (<10 ms), relative large working range (0%–35%) with a maximum gauge factor of 1403.7, followed by good linearity, long-term durability, and the recovery property from being overstretched (>100%). The excellent performance is due to the slippage of the inner graphene under tiny strain, whereas the ‘sewing’ phenomenon of the outer AgNWs under larger strain. The sandwich structure illustrates a better combination of graphene and AgNWs than other hybrid methods, showing great potential in wearable devices and soft robotics.

Enhancement of linearity range of stretchable ultrasensitive metal crack strain sensor via superaligned carbon nanotube-based strain engineering

Linearity range of a crack-based strain sensor is dramatically enhanced while maintaining its ultrahigh mechanosensitivity by introducing an intermediate superaligned carbon nanotube sheet.

Strain sensor for full-scale motion monitoring based on self-assembled PDMS/MWCNTs layers

The combination of strain sensors and wearable devices is an important topic in the field of human–machine interface. We describe a stretchable, flexible, ultrasensitive, simple and low-cost strain sensor based on the PDMS/MWCNTs/PDMS sandwich structure. The sensitive film was prepared by the layer-by-layer self-assembly method. The sensor exhibited the stable mechanical properties in the high measurement range (0.1%–75%) with the gauge factor more than 100. We demonstrate a proof of concept to detect the full-scale change of the human motions, including large movements of human limbs and subtle deformations on human skin. Moreover, the sensor was attached to the throat and face to monitor the human expression during speaking, crying, smiling and swallowing. Due to its excellent characteristics, it could open up a new way for the development of the next generation of high-tech applications, like intelligent devices, wearable electronics and human–machine interface systems.

Ultrasensitive Wearable Strain Sensors of 3D Printing Tough and Conductive Hydrogels.

In this study, tough and conductive hydrogels were printed by 3D printing method. The combination of thermo-responsive agar and ionic-responsive alginate can highly improve the shape fidelity. With addition of agar, ink viscosity was enhanced, further improving its rheological characteristics for a precise printing. After printing, the printed construct was cured via free radical polymerization, and alginate was crosslinked by calcium ions. Most importantly, with calcium crosslinking of alginate, mechanical properties of 3D printed hydrogels are greatly improved. Furthermore, these 3D printed hydrogels can serve as ionic conductors, because hydrogels contain large amounts of water that dissolve excess calcium ions. A wearable resistive strain sensor that can quickly and precisely detect human motions like finger bending was fabricated by a 3D printed hydrogel film. These results demonstrate that the conductive, transparent, and stretchable hydrogels are promising candidates as soft wearable electronics for healthcare, robotics and entertainment.

Two-Sided Topological Architecture on a Monolithic Flexible Substrate for Ultrasensitive Strain Sensors.

Flexible ultrasensitive strain sensors are highly desirable in view of their widespread applications in wearable electronics, health monitoring systems, and smart robots, where subtle strain detection is required. However, traditional fabrication of such sensors was done to prepare sensitive layers on bare or single-sided structural substrates, leading to limited sensitivity. Herein, a stretchable resistive-type strain sensor was demonstrated by self-assembling conductive networks onto a monolithic polydimethylsiloxane substrate with a two-sided topological design, for example, a sinusoid/auxetic binary architecture. The sensitivity of the obtained sensor was greatly improved by 22-fold as compared to the traditional counterpart with a bare substrate. The remarkably good agreement between the experimental results and finite element analysis predictions confirmed that the superior sensitivity is a synergistic effect of local strain enhancement derived from the topological structure on the foreside and an additional strain concentration and a reduced Poisson's ratio from the auxetic arrays on the backside. Furthermore, this sensor can withstand an extreme mechanical force (>750 N) because of the shear stiffening characteristic of the auxetic structure. Benefiting from the characteristics of ultrahigh sensitivity (gauge factor ∼1744 at 5%), low detection limit (<0.05%), and long-term durability (>500 loading cycles), this as-prepared sensor shows promise in practical applications of high-performance wearable electronics.

Ultrasensitive Multifunctional Magnetoresistive Strain Sensor Based on Hair‐Like Magnetization‐Induced Pillar Forests

AbstractAn ultrasensitive multifunctional sensor which integrates tactile sensing and magnetism sensing together in one device is presented. The sensor, dubbed L‐MPF, consists of two interlocking hair‐like magnetization‐induced pillar forests, which are self‐formed under a magnetic field and a loading pressure. An L‐MPF endows intelligent electronics with magnetic field reception, which is the sense of bacteria, birds, and whales, rather than human beings. It is found that the L‐MPF precisely detects the magnitude and the loading path of the dynamic load, including pressure, shear, and magnetic field, with fast response, high reversibility, excellent sensitivity, and high stability. The sensitivity coefficient can reach 1965, 1.6, and 240% T−1 under 50–60% compressive strain, 0–3.4° shear strain, and 21–170 mT magnetic field. Additionally, the strain sensitivity coefficient can be set by the external magnetic field, which affords a potential approach to realize different sensing requirements in various circumstances. For a detailed insight into the sensing element's behavior, the magnetic–mechanic–electric coupling response is also described by simplified model simulation and theoretical analysis. Furthermore, a 4 × 4 L‐MPF array exhibits contactless gesture sensing, which provides a new way for next‐generation human–electronics interface devices and artificial electronic skin.

Low temperature tolerant, ultrasensitive strain sensors based on self-healing hydrogel for self-monitor of human motion

Self-healing hydrogel is attracting growing interest in the field of flexible sensor. However, the inevitably freeze at subzero temperature of conventional self-healing hydrogel significantly limits their further application. A low-temperature tolerance, toughness, conductive, and self-healing hydrogel using water and ethylene glycol as the binary solvent and dynamic supramolecular cross-linking among the carboxyl-decorated reduced graphene oxide (rGO), polyvinyl alcohol and borax is reported here. Owing to the multiple reversible bonds, the hydrogel exhibits excellent self-healing ability (100% in 5 min) at room temperature without any external stimuli. Additionally, the low-temperature tolerance hydrogel can be employed as resistance strain sensor to detect both large and tiny scale motions, which exhibits higher sensitivity (3.26–4.20) than that of conventional metal (2.0) in the temperature range of −10 to 60 °C. The sensor with ability of monitoring multifarious human motions can provide potential applications in healthcare monitoring and human-computer interaction.

Stretchable, Patch-Type Calorie-Expenditure Measurement Device Based on Pop-Up Shaped Nanoscale Crack-Based Sensor.

Demands for precise health information tracking techniques are increasing, especially for daily dietry requirements to prevent obesity, diabetes, etc. Many commercially available sensors that detect dynamic motions of the body lack accuracy, while novel strain sensors at the research level mostly lack the capability to analyze measurements in real life conditions. Here, a stretchable, patch-type calorie expenditure measurement system is demonstrated that integrates an ultrasensitive crack-based strain sensor and Bluetooth-enabled wireless communication circuit to offer both accurate measurements and practical diagnosis of motion. The crack-based strain gauge transformed into a pop-up-shaped structure provides reliable measurements and broad range of strain (≈100%). Combined with the stretchable analysis circuit, the skin attachable tool translates variation of the knee flexion angle into calorie expenditure amount, using relative resistance change (R/R0 ) data from the flexible sensor. As signals from the knee joint angular movement translates velocity and walking/running behavior, the total amount of calorie expenditure is accurately analyzed. Finally, theoretical, experimental, and simulation analysis of signal stability, dynamic noises, and calorie expenditure calculation obtained from the device during exercise are demonstrated. For further applications, the devices are expected to be used in broader range of dynamic motion of the body for diagnosis of abnormalities and for rehabilitation.

Dynamically Modulated GaN Whispering Gallery Lasing Mode for Strain Sensor

AbstractThe continuous development of strain sensors offers significant opportunities for improving human–machine interfaces and health monitoring. The dynamically modulated lasing mode is a novel approach to realize a flexible, noncontact, high color‐resolvability, high‐resolution, and ultrasensitive strain sensor. Here, a flexible strain sensor perceiving stress variations is reported via the dynamical regulation of a GaN whispering gallery lasing mode based on the piezoelectric effect. The refraction index of GaN shows a linear relationship with the applied external tensile strain, resulting in a redshift phenomenon of the lasing mode peak at room temperature due to the predominant function of the piezoelectric polarization in the GaN microwire. Compared with a strain sensor relying on the wavelength shift of a photoluminescence (PL) emission peak, the differences and advantages of a sensor based on the strain‐induced lasing mode variation are also investigated and analyzed systematically. This strain sensor may serve as an essential step toward the color mapping of mechanical signals by optical methods, with potential applications in color‐perceived touching sensing, noncontact stress measurement, laser modulation, and optical communication technologies.

Ultrasensitive strain sensor based on Vernier- effect improved parallel structured fiber-optic Fabry-Perot interferometer.

A novel parallel structured fiber-optic Fabry-Perot interferometer (FPI) based on Vernier-effect is theoretically proposed and experimentally demonstrated for ultrasensitive strain measurement. This proposed sensor consists of open-cavity and closed-cavity fiber-optic FPI, both of which are connected in parallel via a 3 dB coupler. The open-cavity is implemented for sensing, while the closed-cavity for reference. Experimental results show that the proposed parallel structured fiber-optic FPI can provide an ultra-high strain sensitivity of -43.2 pm/με, which is 4.6 times higher than that of a single open-cavity FPI. Furthermore, the sensor is simple in fabrication, robust in structure, and stable in measurement. Finally, the parallel structured fiber-optic FPI scheme proposed in this paper can also be applied to other sensing field, and provide a new perspective idea for high sensitivity sensing.

Dark exciton based strain sensing in tungsten-based transition metal dichalcogenides

The trend towards ever smaller high-performance devices in modern technology requires novel materials with new functionalities. The recent emergence of atomically thin two-dimensional (2D) materials has opened up possibilities for the design of ultra-thin and flexible nanoelectronic devices. As truly 2D materials, they exhibit an optimal surface-to-volume ratio, which results in an extremely high sensitivity to external changes. This makes these materials optimal candidates for sensing applications. Here, we exploit the remarkably diverse exciton landscape in monolayer transition metal dichalcogenides to propose a novel dark-exciton-based concept for ultra sensitive strain sensors. We demonstrate that the dark-bright-exciton separation can be controlled by strain, which has a crucial impact on the activation of dark excitonic states. This results in a pronounced intensity change of dark excitons in photoluminescence spectra, when only 0.05 $\%$ strain is applied. The predicted extremely high optical gauge factors of up to 8000 are promising for the design of optical strain sensors.

Superhydrophobic Electrically Conductive Paper for Ultrasensitive Strain Sensor with Excellent Anticorrosion and Self-Cleaning Property.

Recently, a paper-based (PB) strain sensor has turned out to be an ideal substitute for the polymer-based one because of the merits of renewability, biodegradability, and low cost. However, the hygroexpansion and degradation of the paper after absorbing water are the great challenges for the practical applications of the PB strain sensor. Herein, the superhydrophobic electrically conductive paper was fabricated by simply dip-coating the printing paper into the carbon black (CB)/carbon nanotube (CNT)/methyl cellulose suspension and hydrophobic fumed silica (Hf-SiO2) suspension successively to settle the problem. Because of the existence of ultrasensitive microcrack structures in the electrically conductive CB/CNT layer, the sensor was capable of detecting an ultralow strain as low as 0.1%. During the tension strain range of 0-0.7%, the sensor exhibited a gauge factor of 7.5, almost 3 times higher than that of the conventional metallic-based sensors. In addition, the sensor displayed frequency-independent and excellent durability and reproductivity over 1000 tension cycles. Meanwhile, the superhydrophobic Hf-SiO2 layer with a micro-nano structure and low surface energy endowed the sensor with outstanding waterproof and self-cleaning properties, as well as great sustainability toward cyclic strain and harsh corrosive environment. Finally, the PB strain sensor could effectively monitor human bodily motions such as finger/elbow joint/throat movement and pulse in real time, especially for the wet or rainy conditions. All these pave way for the fabrication of a high-performance PB strain sensor.

The use of complex impedance spectroscopy measurements for improving strain sensor performance

Ultrasensitive strain sensors have significant practical application in human-motion detection. However, there is still a challenge for developing strain sensors that are capable of detecting high strain and a high gauge factor (GF). The aim of this study was to investigate whether a semiconducting polymer and AC measurements are able to improve the GF of stretchable sensors. We employed complex impedance spectroscopy measurements to adjust the optimal operating frequency and the impedance component for improving the device performance of a polyaniline film on a flexible poly(vinyl chloride):diisononyl phthalate sub-strate. In the DC regime, the sensor revealed linearity and GF ∼18 at 46% strain. oreover, the frequency of 100 Hz corresponds to the ideal work re- gion to operate the device that unifies sensitivity and linearity, as well as to directly correlate GF and Z∗(100) data. The most surprising correlation is with GF from Z”(100), i.e. GF”(100), which is about 9 times higher than GF (DC regime) at 46% strain. The Cole-Davidson approach is developed, in which Z’(f) and Z”(f) are calculated as functions of tensile stress. This model provides the correlation between GF”(100) and the electrical resistance and capacitance of the device, as well as the charge-carrier hopping distance dependence on the tensile stress.

Ultra-sensitive flexible strain sensor based on graphene nanocrystallite carbon film with wrinkle structures

Flexible strain sensor is under high demand for detecting the various movements of human, but it is still a challenge to integrate high sensitivity and high stretchability simultaneously in a single flexible strain sensor. Here, based on a multi-scale structure design idea, an ultra-sensitive and highly stretchable strain sensor is fabricated using graphene nanocrystallite carbon film with wrinkle structure. In nano-scale, the intrinsic piezoresistive property of graphene nanocrystallite and the tunneling effect between graphene nanocrystallites bring an ultrahigh gauge factor of 1071. In micro-scale, the strain induced ordered parallel wrinkles and perpendicular microcracks to the strain direction can guarantee the flexible strain sensor stable under a maximum strain of 15%. Application tests of detecting the movement of muscles around eye, throat and wrist are conducted. These application tests prove that this strain sensor has high sensitivity, wide measurement range and rapid responsibility, suggesting a great potential in monitoring human activities.

Ultrastretchable and Stable Strain Sensors Based on Antifreezing and Self-Healing Ionic Organohydrogels for Human Motion Monitoring.

Ionic hydrogels, a class of intrinsically stretchable and conductive materials, are widely used in soft electronics. However, the easy freezing and drying of water-based hydrogels significantly limit their long-term stability. Here, a facile solvent-replacement strategy is developed to fabricate ethylene glycol (Eg)/glycerol (Gl)-water binary antifreezing and antidrying organohydrogels for ultrastretchable and sensitive strain sensing within a wide temperature range. Because of the ready formation of strong hydrogen bonds between Eg/Gl and water molecules, the organohydrogels gain exceptional freezing and drying tolerance with retained deformability, conductivity, and self-healing ability even stay at extreme temperature for a long time. Thus, the fabricated strain sensor displays a gauge factor of 6, which is much higher than previously reported values for hydrogel-based strain sensors. Furthermore, the strain sensor exhibits a relatively wide strain range (0.5-950%) even at -18 °C. Various human motions with different strain levels are monitored by the strain sensor with good stability and repeatability from -18 to 25 °C. The organohydrogels maintained the strain sensing capability when exposed to ambient air for nine months. This work provides new insight into the fabrication of stable, ultrastretchable, and ultrasensitive strain sensors using chemically modified organohydrogel for emerging wearable electronics.

Ultrasensitive Strain Sensor Based on Separation of Overlapped Carbon Nanotubes

Although there have been remarkable improvements in stretchable strain sensors, the development of strain sensors with scalable fabrication techniques and which both high sensitivity and stretchability simultaneously is still challenging. In this work, a stretchable strain sensor based on overlapped carbon nanotube (CNT) bundles coupled with a silicone elastomer is presented. The strain sensor with overlapped CNTs is prepared by synthesizing line-patterned vertically aligned CNT bundles and rolling and transferring them to the silicone elastomer. With the sliding and disconnection of the overlapped CNTs, the strain sensor performs excellently with a broad sensing range (≥145% strain), ultrahigh sensitivity (gauge factor of 42 300 at a strain of 125-145%), high repeatability, and durability. The performance of the sensor is also tunable by controlling the overlapped area of CNT bundles. Detailed mechanisms of the sensor and its applications in human motion detection are also further investigated. With the novel structure and mechanism, the sensor can detect a wide range of strains with high sensitivity, demonstrating the potential for numerous applications including wearable healthcare devices.

Ultrasensitive and wearable strain sensors based on natural rubber/graphene foam

Recently, flexible strain sensors have attracted much attention, but there is still a challenge to manufacture flexible strain sensors with superb sensitivity and a large workable strain range simultaneously. Herein, a simple, rapid, and low-cost stratagem can be produced to fabricate novel strain sensors based on reduced graphene oxide (GO) foam in large quantities scalable. The PU sponge was used as a template to adsorb GO, and graphene foams (GF) was synthesized by pyrolyzing PU @ GO by the alcohol flame. Then the NR/GF composite was prepared by casting Natural Rubber (NR) latex into GF. The results showed that the gauge factor of the flexible sensor based on NR/GF composite reached 210 in the strain range of 10–40%. It could be used to detect motion, such as exercise steps, finger bending and identification strain types. Therefore, the NR/GF composites sensor has broad prospects of development in intelligent human-computer interaction, medical and other fields.

Spider-Web-Inspired Stretchable Graphene Woven Fabric for Highly Sensitive, Transparent, Wearable Strain Sensors.

Advanced wearable strain sensors with high sensitivity and stretchability are an essential component of flexible and soft electronic devices. Conventional metal- and semiconductor-based strain sensors are rigid, fragile, and opaque, restricting their applications in wearable electronics. Graphene-based percolative structures possess high flexibility and transparency but lack high sensitivity and stretchability. Inspired by the highly flexible spider web architecture, we propose semitransparent, ultrasensitive, and wearable strain sensors made from an elastomer-filled graphene woven fabric (E-GWF) for monitoring human physiological signals. The highly flexible elastomer microskeleton and the hierarchical structure of a graphene tube offer the strain sensor with both excellent sensing and switching capabilities. Two different types of E-GWF sensors, including freestanding E-GWF and E-GWF/polydimethylsiloxane (PDMS) composites, are developed. When their structure is controlled and optimized, the E-GWF strain sensors simultaneously exhibit extraordinary characteristics, such as a high gauge factor (70 at 10% strain, which ascends to 282 at 20%) in respect to other semitransparent or transparent strain sensors, a broad sensing range up to 30%, and excellent linearity. The E-GWF/PDMS composite sensor shows a unique reversible switching behavior at a high strain level of 30-50%, making it a suitable material for fast and reversible strain switching required in many early warning systems. With a view to real-world applications of these sensors and switches, we demonstrate human motion detection and switch controls of light-emitting-diode lamps and liquid-crystal-display circuits. Their unique structure and capabilities can find a wide range of practical applications, such as health monitoring, medical diagnosis, early warning systems for structural failure, and wearable displays.

Bioinspired and bristled microparticles for ultrasensitive pressure and strain sensors

Biological sensory organelles are often structurally optimized for high sensitivity. Tactile hairs or bristles are ubiquitous mechanosensory organelles in insects. The bristle features a tapering spine that not only serves as a lever arm to promote signal transduction, but also a clever design to protect it from mechanical breaking. A hierarchical distribution over the body further improves the signal detection from all directions. We mimic these features by using synthetic zinc oxide microparticles, each having spherically-distributed, high-aspect-ratio, and high-density nanostructured spines resembling biological bristles. Sensors based on thin films assembled from these microparticles achieve static-pressure detection down to 0.015 Pa, sensitivity up to 121 kPa−1, and a strain gauge factor >104, showing supreme overall performance. Other properties including a robust cyclability >2000, fast response time ~7 ms, and low-temperature synthesis compatible to various integrations further indicate the potential of this sensor technology in applying to wearable technologies and human interfaces.